THE RELATIONSHIP BETWEEN TENSION AND SLOWLY VARYING INTRACELLULAR CALCIUM-CONCENTRATION IN INTACT FROG SKELETAL-MUSCLE

1. The relationship between intracellular calcium concentration, [Ca2](i), and fixed-end tension was investigated in intact single muscle f ibres from frogs. A slow decline of tension was produced by cyclopiazo nic acid (CPA), a sarcoplasmic reticulum Ca2+ uptake pump inhibitor. T he fluorescent dyes fura-2 and furaptra (mag-fura-2) were used to esti mate [Ca2+](i). 2. Neither the steepness nor the position of the curve changed consistently over a wide range of tension decay times from a few seconds to over 100 s. For these near-steady-state curves, the 10- 90% tension change occurred, on average, in 0.07 pCa units, correspond ing to a Hill coefficient > 25, much steeper than previously reported. Possible artifacts could reduce that to 15. 3. Methoxyverapamil (D600 ) reduces the calcium released in response to an action potential. Con tractions with D600 and CPA had a slow rise composed of many small ste ps, and a slow fall. Comparing rise and fall showed little or no hyste resis in the tension-[Ca2+](i) relationship. 4. A model involving co-o perativity between the binding of Ca2+ and myosin to thin filaments is shown to produce a tension-pCa relationship that is substantially alt ered by the mean rate constant for detachment of myosin cross-bridges, which in turn is likely to be affected by sarcomere movements. 5. Suc h a model is shown to be capable of reproducing the small rise in [Ca2 +](i) previously reported during the late phase of fixed-end relaxatio n of intact fibres.